Project Summary/Abstract There is a fundamental gap in understanding how neuronal function or dysfunction in specific inhibitory popula- tions in mammalian central nervous system translates into normal or altered behavior. Natural behavior, often expressed as movement, is generated by excitatory networks whose function is shaped by inhibitory ?-amino- butyric acid (GABA)-ergic and glycinergic neurons. Inhibitory dysfunction underlies a number of disabling neu- rodevelopmental disorders?many, e.g., Rett syndrome, with comorbid motor disturbances. Determining how inhibition shapes basic motor programs represents a strategy for understanding both normal network function and how malfunctioning networks might be repaired/treated clinically. Among basic motor behaviors, only for breathing has a localized rhythmogenic network, the preBtzinger Complex (preBtC), been identified. Within the preBtC are GABAergic and glycinergic neurons, presenting an inimitable opportunity to study the role of inhibition; that this can be done in a slice in vitro presents considerable technical advantages. Current approa- ches for studying inhibition extrapolate from small samples or ignore important heterogeneity within neuronal populations. Overlooked are inhibitory microcircuits?local, embedded networks of GABAergic and glycinergic neurons that target nearby inhibitory and excitatory neurons. Two long-standing obstacles to addressing the role of inhibitory microcircuits are the dynamic complexity that can emerge in neuronal networks and the inabi- lity to dynamically manipulate inhibitory microcircuits. To overcome these obstacles, we combine a conceptu- ally innovative approach, focused on minimal microcircuits, with a technically innovative solution, holographic photostimulation, capable of exciting or inhibiting specific groups of neurons within a population with excep- tional spatiotemporal resolution. Using these approaches in rhythmic medullary slices from transgenic mice, we test our central hypothesis that synaptic and network properties determine how preBtC inhibitory microcircuits control the dynamic repertoire of respiratory-related behaviors in three specific AIMS. In AIM 1, we determine how preBtC inhibitory microcircuits shape respiratory output. In AIM 2, we determine how synaptic and net- work mechanisms underlie the effects of preBtC inhibitory microcircuit activation. In AIM 3, we determine how microcircuit-microcircuit interactions expand the dynamic repertoire of the preBtC. The contribution of the proposed research is expected to be elucidation of specific mechanisms underlying control of breathing by pre- BtC inhibitory microcircuits. This contribution is significant because determining these mechanisms is neces- sary for understanding how inhibitory microcircuits shape breathing in health and disease and are themselves regulated as targets of other circuits to generate complex respiratory-related behaviors. The overall impact of this proposal will be to reveal basic neural circuit mechanisms controlling a vital behavior, demonstrate the potential of dynamic patterned manipulations for dissecting neural circuits, pave the way for understanding more complex behaviors, and possibly uncover general principles of inhibitory microcircuit function.